Modern integrated circuits (IC) are complex devices and it has become increasingly difficult to fabricate all of the required circuitry for such devices on a single die. As a result, complex circuits are often created from circuit components in two or more dies and the dies are connected together through electrical connections. One such connection technology is a so-called through-silicon via (TSV) which is a form of three dimensional (3D) interconnection between circuits in wafers or dies stacked on top of one another. U.S. Pat. No. 7,157,787 describes one such method of connecting stacked wafers or dies by making a face-to-face contact between the wafers/dies and creating thru vias to add additional stacked pairs.
In any 3D interconnect it is important to ensure good electrical contact between the components thereof. To effect reliable connections between contact elements (e.g., bonding pads) of two semiconductor dies, one must be concerned with several parameters including, but not limited to: alignment of the dies and contact pads, contact force, balanced contact force, contact resistance, and planarization. Wafer/die alignment and planarization (i.e., flatness) requirements pose serious challenges as wafers/dies are often warped during manufacturing process (e.g., due to different temperature profiles across the wafers/dies and the many different materials used therein). Complicating the problem, one must also take steps to ensure good ohmic contact is made between the circuits disposed on the individual wafers/dies.
U.S. Pat. No. 6,975,016 proposes the use of a flexible bladder behind a thinned wafer to address wafer/die flatness and enable good contact between two wafers prior to bonding. U.S. Pat. No. 5,884,398 describes a method of adding separately manufactured, resilient spring contacts onto the bond pads on the dies to provide electrical connections between two stacked dies. Adding such spring contacts allows for testing the dies before dicing the wafer.